There are many forms of printers for printing a dot-matrix image on a print medium, which most typically is a sheet of paper or transparent film. Two of these involve the use of a rotating drum that supports the print medium. A print head is moved along the surface of the drum parallel to the axis of rotation of the drum. As used herein, the term "X-axis motion" refers to movement of the print head in a direction parallel to the drum's axis of rotation, which conventionally is horizontal. "Y-axis motion" refers to movement in the direction of rotation of the drum, and thus corresponds to movement of the print head around the circumference of the drum.
In both of these forms of printers, the print head advances in one direction slowly, usually in increments, while scanning rapidly in the other direction. As used herein, "direction" refers to motion in either direction along a line. The direction in which the print head advances slowly is referred to herein as the advance direction. The direction in which the print head scans rapidly is referred to as the scan direction. Scanning takes place typically by either reciprocating (shuttling) the print head in X-axis motion, or by rotating the drum rapidly resulting in relative print head Y-axis motion.
The preferred method and embodiment for practicing the present invention is particularly directed to a hot-melt ink jet printer wherein a print head advances in increments in X-axis motion which is therefore the advance direction, while a sheet of paper or film is held against a rotating drum. Images are formed by selectively and serially depositing ink drops of primary or base colors at uniformly spaced address locations to form a dot-matrix image. The present invention however is also applicable to any printing process wherein a print head travels along the scan axis relative to a print medium to form a desired final graphic or textual image formed of spaced dots or image elements.
An example of a printer with a reciprocating print head is disclosed in U.S. Pat. No. 4,864,328 issued to Fischbeck. That printer advances the print medium at one speed for low-quality printing and at a lower speed for high-quality printing. Since adjacent, pixels are printed in both the advance and scan directions, ink drops are deposited at a maximum rate, but no interlacing of the ink drops is achieved.
Print heads are also known that contain an array of nozzles that are spaced and of a predetermined number to provide interlaced printing with each image element being addressed once by only one nozzle. Such a technique for a single color is described by Paranjpe et al. in U.S. Pat. No. 4,112,469. In column 7 of that patent, a formula is provided for determining the spacing and number of nozzles that may be used to provide interlaced printing in the advance direction. Variations in the layout of nozzle arrays that achieve such interlaced printing are also illustrated in U.S. Pat. No. 4,232,324 issued to Tsao.
In each of such printing devices, the interlacing is in the advance direction only, with image rows or columns being printed successively. This means that every printed pixel or image-element location aligned with a print orifice during a scan must have an ink drop deposited on it during that scan. Adjacent pixels are therefore printed at close to the same time. For some inks, such as aqueous and hot-melt inks, there is a mixing of the adjacent ink drops. Mixing of aqueous inks does not result in a different appearance, particularly if they are the same color.
However, if dots of hot-melt ink that have not solidified are deposited adjacent to or on top of one another, they mix. When they mix, the resultant color is different than it is if the first dot solidifies before the second dot is deposited. Part of the reason for this is that the hot-melt ink has a surface texture that is different when the adjacent ink drops mix, resulting in a different optical appearance.
One printing pattern that avoids printing adjacent pixels during a single scan is the color ink jet printer sold under the proprietary name PaintJet XL by Hewlett Packard. This printer prints a checkerboard pattern on each pass of the print head over a print medium, by printing only alternate pixel locations in both directions. The print medium is then shifted and a checkerboard pattern offset from the previous pattern is printed so that all pixel locations in the overlapped region are addressed in two passes.
If this technique is applied to hot-melt ink printers, the mixing between dots is substantially avoided. However, this and other conventional printers use the individual printing elements or nozzles at less than their maximum efficiency. The Hewlett Packard system requires two sets of nozzles to fully address all the pixel locations. One set prints while the carriage scans in a positive direction, and the other set prints while it moves in the negative direction. Such interlaced printing provides a high quality image but results in substantial underutilization of the print head compared to what continuous printing by the same number of nozzles could theoretically provide.